Weakly and tightly bound polarons in the infrared spectra of perovskites

Infrared reflectivity and/or absorption measurements in chemically doped perovskites, including high-T_c superconductors, show polaronic features similar to those observed in the photoinduced spectra of cuprates. These features, whose intensities depend on doping and temperature, are observed in both n-type and p-type materials. They include extra-phono infrared-active vibrations (IRAV) and a broad band, centered at ~ 1000cm^-1, which results from overtones of IRAVs and is well described in terms of photoexcitation of polarons. Most of the above features survive as extra-Drude peaks in the metallic phase of these materials. The strength of the polaron band increaseing temperature, as expected for weakly bound polarons, in compounds which are high-T_c superconductors. It decreases, instead, as expected for tightly bound polarons, in a Mn-based perovskite and in CuO, which do not become superconducting upon doping. A comparison with tunneling results shows that the superconducting carriers are strongly coupled to the IRAVs.